A prior art printing apparatus shown in FIG. 4 comprises an anilox roll (ink roll) 1; a rubber roll (another ink roll) 17; a nip 2 formed between the anilox roll 1 and the rubber roll 17; an ink pump 11; an ink supply duct 12; ink discharge ducts 13 and 13'; ink reservoirs 14a and 14b; ink supply ports 15 formed in the ink reservoirs 14a and 14b for connection to said ink supply duct 12; ink recovery ports 16 formed in the ink reservoirs 14a and 14b for connection to said ink discharge duct 14; a flexo ink 18 contained in said ink reservoirs 14a and 14b; end pans (ink sumps) 19 and 19' provided beneath the opposite ends of said anilox roll 1 and said rubber roll 17; a bucket 20; an electromagnetic valve 21 of a washing device; and a washing liquid 22. The flexo ink 18 is supplied from the ink reservoir 14a to the nip 2 between the anilox roll 1 and the rubber roll 17 through the ink supply port 15, through the ink supply duct 12, through the ink pump 11, and then again through the ink supply duct 12, as indicated by solid line arrows in the figure. The flexo ink 18 supplied to the nip 2 is doctored by the rubber roll 17 and is homogeneously transferred to the surface of the anilox roll 1 and is then further transferred to the printing plate of a printing cylinder (not shown). A corrugated fiberboard sheet is brought into contact with the printing plate of the printing cylinder, whereby the sheet is printed. During such printing process, the flexo ink 18 is continuously supplied to the nip 2. Excess quantities of the flexo ink 18 will overflow both ends of the nip 2, flowing down into the end pans (ink sumps) 19 and 19'. The ink is then returned to the ink reservoir 14a through the ink discharge ducts 13' and 13 and through the ink recovery port 16. The flexo ink 18 contained in the ink reservoir 14a is continuously supplied to the nip 2 between the anilox roll 1 and the rubber roll 17 via the above indicated path while said corrugated fiberboard sheet is printed.
The ink can be replaced by other inks for meeting order changes, for example, in a manner as follows: First, the ink pump 11 is deactivated to stop the flow of the flexo ink 18 through said ink circulating path. An exchanger mechanism (not shown) is then actuated in order to remove the lower ends of the ink supply duct 12 and of the ink discharge duct 13 from, and then position them immediately above, the ink supply port 15 and the ink recovery port 16, respectively, of the ink reservoir 14a. Such positioning is maintained until a preset time set in a timer (not shown) elapses. In the meantime, the quantities of the flexo ink 18 remaining in the nip 2 or in the ink circulating path is allowed to flow down into the ink reservoir 14a under the influence of the gravity, whereby the ink can be recovered. After the preset time set in the timer has elapsed and the remaining quantities of the flexo ink 18 in the nip 2 and in the ink circulating path have been collected in the ink reservoir 14a, the timer will give an ink recovery completion signal to the exchanger mechanism. In response to the signal, the mechanism will operate such as to move the lower ends of the ink supply duct 12 and of the ink discharge duct 13 to their respective predetermined positions over the bucket 20. Then, the electromagnetic valve 21 of the ink washing device is opened to allow the washing liquid 22 to flow through the electromagnetic valve 21 toward the section of the ink supply duct 12 where the ink pump 11 is located, on the one hand, and toward the ink reservoir 14a through another section of the ink supply duct 12 which extends toward the nip 2, through the nip 2, through the end pans (ink sumps) 19 and 19', through the ink discharge ducts 13 and 13', on the other hand, as indicated by broken line arrows, whereby the nip 2 and the ink circulating path are washed, and the used washing liquid 22 is collected in the bucket 20. Upon the lapse of a predetermined time, the electromagnetic valve 21 is closed to stop the supply of the washing liquid 22. The conditions are maintained until a preset time set in the timer elapses; meanwhile, the remaining quantities of the washing liquid 22 in the nip 2 and in the ink circulating path are allowed to flow down into the bucket 20 under the influence of the gravity. When the preset time set in the timer has elapsed and the remaining quantities of the washing liquid 22 in the nip 2 and in the ink circulating path have been discharged into the bucket 20, the timer will give a washing liquid recovery completion signal to the exchanger mechanism. In response to the signal, the exchanger is actuated such as to connect the lower ends of the ink supply duct 12 and of the ink discharge duct 13 to the ink supply port 15 and the ink recovery port 16, respectively, of another ink reservoir 14b containing therein the next order flexo ink 18. Then, the ink pump 11 is actuated to supply the flexo ink 18 from the ink reservoir 14b to the nip 2 between the anilox roll 1 and the rubber roll 17 through the ink supply port 15, through ink supply duct 12, through the ink pump 11, and the again through the ink supply duct 12.
In the prior art printing apparatus shown in FIG. 4, the flexo ink 18 or washing liquid 22 remaining in the nip 2 or in the ink circulating path is recovered by their spontaneous flowing down movement. It thus takes a relatively long time to recover the flexo ink 18 or the washing liquid 22; this is a cause of low productive efficiency. Further, small quantities of flexo inks 18 will always inevitably remain in the nip 2 and in the ink circulating path even after the spontaneous flowing down since flexo inks are viscous, and such remaining inks will consequently be mixed with the next order flexo ink 18, causing a problem of deteriorating the quality of the print.